Amplifier



M. ARTZT July 13, 1943.

AMPLIFIER .Filed Oct. '7, 1938 Cl/OPPIR 70/75 INPUT 2.5. ourPuT "w n b vi J H HOPPER 70/1/15 lAPU 7' W Y Ri XJ M m .m W% VM M ma" Patented July13, 1943 AMPLIFIER Maurice Artzt, Haddoniield, N. J., assignor to RadioCorporation of America, a corporation of Delaware Application October 7,1938, Serial No. 233,701

1 Claim.

The present invention relates to amplifiers and particularly toamplifiers of the so-called direct current type.

It has been known in the art that the direct current type of amplifieris desirable for many purposes, such, for example, as to amplifyrelatively weak photoelectric tube outputs and for measurement purposes,but in many instances it has been diflicult to obtain good stability orefficient operation'from such type of amplifier systems. Unstable andinefiicient operation of such amplifiers makes their use undesirable, asa general rule, for the purposes set forth, but where it is possible toprovide an amplifier of such character which is substantially absolutelystable in its operation there is wide application for its use in thetransmission of facsimile representations as well as for the use formeasuring devices and other general applications.

According to the present invention there has been provided an inherentlystable direct current amplifier system capable of amplifying smalldirect current voltages. Such an amplifier has its non-driftcharacteristics made possible by the use of alternating currentcouplings throughout and by the provision of a suitable means forchopping the direct current input at a frequency considerably higherthan the frequency of any variation in the direct current signal whichis to be amplified. With such an arrangement of circuit elements theoutput of the alternating current amplifier is a tone frequencymodulated by the direct current with the amplitude of the modulated tonevarying in direct proportion to the direct current I input to thesystem. Many and various ways may be provided for providing thechopping" action abovementioned but one suitable arrangement which willbe particularly described herein is an impedance changing bridgenetwork. When the impedance chan ing bridge network to which the directcurrent energy is applied causes the development of the modulated tonesuch tone may later be rectified to obtain thereby the amplified directcurrent signal.

From the foregoing it can be appreciated that one of the main objects ofthis invention is that of providing a conveniently operating and emcientmeans for amplifying direct currents.

Other objects of the invention are those of providing electricalnetworks for amplification purposes which shall be stable in theircharacteristic and capable, at the same time, of amplifying smallcurrents Without drift.

Still other objects of the invention are those of providing anelectrical network for the above stated purposes which is relativelysimple in its arrangement of parts and which may be set up and operatedin a minimum amount of time and at a minimum of expense, I

Other objects and advantages of the invention will naturally becomeapparent-and at once suggest themselves to those skilled in the art towhich the invention is directed by reading the following specificationand the accompanying claim in connection with the drawing illustratingtypical circuit arrangements wherein:

Fig. 1 represents one form of the amplifier system which is particularlyadapted to the amplification of photo-electric currents in a facsimilesystem; and,

Fig. 2 is a somewhat simplified single side system which is particularlysuitable and useful for measurement purposes.

Referring now to the accompanying drawing for a further understanding ofthe invention, and first to Fig. l thereof, the photoelectric tube IIwhich is subjected to light of varying intensities in thetranslation ofthe light values of a subject into electric current impulses for theproduction of facsimiles, for example, (all as is well known in the art)is so connected that'its anode element l3 connects to the positiveterminal of a source l5 having its negative terminal preferably atground potential through the indicated connection at IT. The cathode orlight sensitive element IQ of the phototube H is connected to thepositive end of the resistor element 2|, which has its negative terminalconnected to the ground potential point ll.

At the terminal points 23, 23 there is supplied a chopper tone frequencyof suitable value which is impressed upon the amplifier through thetransformer 25 which has its secondary Winding 26 center tapped toground I! by the connection 21. In this way the tone impressed on theterminals 23 is supplied to each of the control electrodes 28 and 29 ofa pair of sw tching tubes 30 and 3|, in phase opposition. The plate oranode elements 32 and 33 of the tubes 30 and 3i connect with one end ofresistor elements 34 and 35 which have'their opposite ends connected tothe positive terminal of the resistor 2!.

The tone frequency supplied to the control electrodes 28 and 29 of thetubes 30 and 3| is fed from the opposite ends of the transformersecondary wind ng 26 by way of the resistor elements 36 and 31 and isthus applied to the tubes 30 and 3i in phase opposition.

Output energy from the tubes 30 and 3| is fed by way of the directcurrent connection 38 and 39 to energize the control electrodes 40 and I4| of the tubes 42 and 43. Output energy from the plate oranode'elements 44 and 45 of the tubes 42 and 43 is supplied to the anodecircuits of these tubes so as to be fed through the primary winding 46of transformer 41 to energize the secondary circuit 48 thereof fromwhich the modulated output of the phototube H is supplied to any desiredform of utilization circuit or transmitter (not shown).

Suitable operating potentials for the tubes 42 and 43 are supplied froma suitable source of voltage connected between the terminal points 49and 50 with the positive terminal of the source connected at theterminal 49. Cathode bias for the cathode elements and 52 of tubes 42and 43 is supplied through the resistor element 53 shunted by theby-pass condenser 54.

In the circuit described above it will be seen that the direct currentoutput of the phototube appears across the resistor 2| which ispolarized as indicated by the drawing. The switching tubes 30 and 3| areso connected as to use this voltage across resistor 2| as the source ofanode voltage through the series resistors 34 and 35. The chopper tonefrequency applied to the control electrodes 28 and 29 of these tubes 30and 3| serves alternately to drive these tubes to saturation and tocut-oil so that the plate impedance of these tubes is alternatelysubstantially infinite and very low in comparison to the resistors 34and 35. Thus, when the tube 36 has infinite plate impedance the tube 3|has very low impedance, Under such'circumstances the voltage upon thecontrol electrode 4| of the tube 43 is effectively short circuited bythe tube 3! and the full direct current voltage across the resistor 2|appears upon the control electrode 46 of tube 42. On the next half cycleof the chopper tone frequency input upon the terminals 23 the grid orcontrol electrode voltage on the tube-42 will be effectively shortcircuited so that the full direct current voltage across the resistor 2|will be impressed upon the control electrode 4| of the tube 43.

The input to the tubes 42 and 43 will, therefore, be rectangular shapedwaves of direct current on alternate half cycles; the amplitude of theapplied voltage being proportional to the direct current input uponthe-system and the duration of the pulses determined by the chopper tonefrequency.

This output voltage can readily be amplified by an alternating currentamplifier system as a modulated wave and later rectified, if desirable.

Under such circumstances the chopper tone frequency may be adjusted bysuitable means (not shown) so as to send facsimile signals as anamplitude modulated wave.

By the arrangement of Fig. 2, there is shown a modified circuit of aconsiderably simplified nature to work as a so-called one side systemwhich is particularly useful for measurement work. In this arrangementof the system, the direct current input, which may be such as thephotoelectric tube ll of Fig, 1 or other suitable source, is supplied atthe terminal points 5| between which is connected the load resistor 2|,as is the arrangement of Fig. 1.. In this way anode or plate voltage forthe tube 30 is provided through the resistor 34 and the tube is causedto become conducting under the control of the chopper tone frequencysupplied to the control electrode or grid 3| of the tube 36 by way ofthe secondary winding 26 of transformer 25 and the resistor 36.

The anode 32 of tube 30 is connected to one side of a coupling condenser53 whose opposite terminal connects to the control electrode or gridelement 55 of the tube 56, across whose input circuit is the leakresistor 51. The cathode 58 of the tube 56 is biased by the cathoderesistor 59 which is shunted by the by-pass condenser 60. The outputenergy is fed by way of the resistance coupling which includes the plateor anode resistor 6|, the coupling condenser 62 and the leak resistor 64to the rectifier 66. The output energy from the rectifier 66 is suppliedacross the output or load resistor 68, suitably shunted by the bypasscondenser 69, to the output terminals 10, 10.

In the arrangement of Fig. 2, the direct current input is applied to theterminals 5| which connect across the resistor 2|. Such an input may beof the same general type as that disclosed by Fig. 1 or any of thevarious known types of direct current input and variable intensity asdesired and in accordance with varying values of light, current, voltageand the like.

With chopper tone applied at terminals 23 the tube 30 is causedalternately to become conducting and non-conducting. When tube 30 isconducting it derives its plate potential from the potential drop acrossresistor 2| which represents the input signal. When conducting, tube 30acts as a low impedance across the network consisting of the capacitor53 and the resistor 51, effectively short-circuiting the input to thisnetwork. When non-conducting, the input to 63 and 51 is raised to thelevel of the potential drop across resistor 2|. Thus the input to 53 and61 is alternately equal to the signal across 2| and then zero, and thecontrol grid 55 follows this alternating off-on signal.

The coupling network 6|, 64 and 62 couples this square wave output oftube 56 back to ground level, where it is rectified by diode 66 toobtain the direct current signal envelope. This appears as a potentialacross resistor 68 and capacitor 69.

Thus, with plate current flowing through the rectifier 66 the condenser69, which is included in the integrating circuit including resistor 68,becomes charged and the magnitude of the charge acquired to beappreciated will be proportional to the direct current input voltageapplied to the terminals 5|. During the inoperative periods of the tubes66 the condenser will discharge to a predetermined reference value(determined by chosen circuit parameter) after which the charge will,during the next half cycle of the chopper tone input, be built up andapplied at 23 to the output terminals 10 as before.

From the foregoing, it is apparent that many modifications and changesmay be made provided and, accordingly, various modifications may be madewithout departing from the spirit and scope of the invention providedsuch modifications and changes fall fairly within the spirit and scopeof the claim hereinafter appended.

What I claim is:

An amplifier system comprising a light translating element serving as asource of variable and direct current, a pair of thermionic tubes eachhaving a cathode, anode and at least one control electrode, a separateload impedance associated with each of the thermionic tubes, each ofsaid load impedances having one terminal connected to the anode elementof the associated thermionic tube and the other end connected directlyto receive the output energy from the light translating element wherebythe voltage instancontrol electrode, a direct current connection bephaserelationship, said tone frequency being of such value as to drive thesaid tubes alternately between cutofi and saturation, a pair of pushpullconnected thermionic output tubes each comprising a cathode, anode andat least one tween the anode electrode of each of the first named pairof tubes and a control electrode of each of the second named pair oftubes, whereby the control electrode of each of the second named pair oftubes has its potential influenced by the output voltage from the lighttranslating element and the potential of the control electrodes of thepush-pull tubes is caused to vary between relatively wide limits by theapplied tone frequency energy, and a load circuit connected to receivethe output of said tubes.

' MAURICE AR'IZT.

